The invention relates to a method for producing components from lightweight steel.
In the following the production of components is described, which were for example generated from strips, sheets or tubes by forming and which are for example used in the field of machine construction, plant construction and ship construction and in particular in vehicle construction for example for vehicle body parts and chassis parts.
Especially the hotly contested automobile market forces manufacturers to constantly seek solutions to lower fleet consumption while retaining highest possible comfort and occupant safety. In this regard on one hand weight saving of all vehicle components plays an important role but on the higher hand also properties of the individual components that promote the passive safety for passengers under conditions of high static and dynamic stress during operation and in the event of a crash.
Hereby the individual components have to meet very different requirements regarding strength, tenacity, wear resistance etc. An example for this are on one hand airbag mounts, which have to possess a very high tenacity in order to be able to absorb the energy introduced in the event of abrupt stress. On the other hand, for example in the case of transverse or longitudinal members of motor vehicles, high strengths have to be achieved also in regions that are formed to a lesser, wherein also a sufficiently high tenacity of the components has to be ensured.
In order to be able to achieve these sometimes contrary component properties, beside using classic austenitic chromium nickel steels new material concepts have been developed, which are optimally tailored to the respective demands placed on the component. These include for example duplex or multiphase steels, air-hardened steels or recently high-manganese-content austenitic lightweight steels.
A disadvantage is however that alloy concepts, which are adapted to the respective demands and are oftentimes expensive, have to be used for producing the components. Until now it has not been possible to satisfy different demands with only one material. In lightweight steels significant progress has been made in recent years. These steels are characterized by a low specific weight while at the same time having a high strength and tenacity with a high ductility, which makes them very interesting for vehicle construction (for example EP 0 489 727 B1, EP 0 573 641′ B1, DE 199 00199 A1).
In these steels, which are austenitic in the starting state, the high proportion of alloy components with a specific weight far below the specific weight of iron (Mn, Si, Al) achieves a weight reduction, which is advantageous for vehicle construction, while at the same time retaining the usual construction.
From DE 10 2004 061 284 A1 for example a lightweight steel is known with an alloy composition (in weight %):
C0.04up to 1.0Al0.05up to <4.0Si0.05up to 6.0Mn9.0up to <18.0remainder iron including usual steel accompanying elements. Optionally depending on the requirements Cr, Cu, Ti, Zr, V and Nb can be added.
This known lightweight steel has a partially stabilized mixed-crystal microstructure with defined stacking fault energy with a partially multiple TRIP effect, which the tension or stretch induced transformation of a face-centered mixed crystal (austenite into a martensite (hexagonal highest density spherical packing) which then during further deformation transforms into a body-centered martensite and residual austenite. The high degree of deformation is achieved by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) properties of the steel.
Numerous tests have shown that the carbon content is of paramount importance in the complex interaction between Al, Si and Mn. On one hand it increases the stacking fault energy and on the other hand expands the metastable austenite region. As a result the deformation-induced martensite formation and the associated hardening and also the ductility can be influenced over broad ranges.
With these lightweight steels many customer demands can already be satisfied to the most degree, however, there is still the desire to produce stress-optimized components made of lightweight steel with smallest possible alloy costs and at the same time satisfying different demands corresponding to the expected stress during operation regarding strength, tenacity, wear-resistance etc. However, this demand can currently only be met by steels having alloy compositions that are adapted to the respective demands and is therefore associated with increased manufacturing costs.